Change the way the offset of downlink is stored in GISTInsertStack.

GISTInsertStack.childoffnum used to mean "offset of the downlink in this
node, pointing to the child node in the stack". It's now replaced with
downlinkoffnum, which means "offset of the downlink in the parent of this
node". gistFindPath() already used childoffnum with this new meaning, and
had an extra step at the end to pull all the childoffnum values down one
node in the stack, to adjust the stack for the meaning that childoffnum had
elsewhere. That's no longer required.

The reason to do this now is this new representation is more convenient for
the GiST fast build patch that Alexander Korotkov is working on.

While we're at it, replace the linked list used in gistFindPath with a
standard List, and make gistFindPath() static.

Alexander Korotkov, with some changes by me.

Author: hlinnaka

src/backend/access/gist/gist.c

@@ -626,6 +626,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 	firststack.blkno = GIST_ROOT_BLKNO;
 	firststack.lsn.xrecoff = 0;
 	firststack.parent = NULL;
+	firststack.downlinkoffnum = InvalidOffsetNumber;
 	state.stack = stack = &firststack;
 
 	/*
@@ -702,9 +703,10 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 			BlockNumber childblkno;
 			IndexTuple	newtup;
 			GISTInsertStack *item;
+			OffsetNumber downlinkoffnum;
 
-			stack->childoffnum = gistchoose(state.r, stack->page, itup, giststate);
-			iid = PageGetItemId(stack->page, stack->childoffnum);
+			downlinkoffnum = gistchoose(state.r, stack->page, itup, giststate);
+			iid = PageGetItemId(stack->page, downlinkoffnum);
 			idxtuple = (IndexTuple) PageGetItem(stack->page, iid);
 			childblkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
 
@@ -754,7 +756,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 				 * tuple.
 				 */
 				if (gistinserttuples(&state, stack, giststate, &newtup, 1,
-									 stack->childoffnum, InvalidBuffer))
+									 downlinkoffnum, InvalidBuffer))
 				{
 					/*
 					 * If this was a root split, the root page continues to be
@@ -778,6 +780,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 			item = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
 			item->blkno = childblkno;
 			item->parent = stack;
+			item->downlinkoffnum = downlinkoffnum;
 			state.stack = stack = item;
 		}
 		else
@@ -854,29 +857,37 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 /*
  * Traverse the tree to find path from root page to specified "child" block.
  *
- * returns from the beginning of closest parent;
+ * returns a new insertion stack, starting from the parent of "child", up
+ * to the root. *downlinkoffnum is set to the offset of the downlink in the
+ * direct parent of child.
  *
  * To prevent deadlocks, this should lock only one page at a time.
  */
-GISTInsertStack *
-gistFindPath(Relation r, BlockNumber child)
+static GISTInsertStack *
+gistFindPath(Relation r, BlockNumber child, OffsetNumber *downlinkoffnum)
 {
 	Page		page;
 	Buffer		buffer;
 	OffsetNumber i,
 				maxoff;
 	ItemId		iid;
 	IndexTuple	idxtuple;
+	List	   *fifo;
 	GISTInsertStack *top,
-			   *tail,
 			   *ptr;
 	BlockNumber blkno;
 
-	top = tail = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
+	top = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
 	top->blkno = GIST_ROOT_BLKNO;
+	top->downlinkoffnum = InvalidOffsetNumber;
 
-	while (top && top->blkno != child)
+	fifo = list_make1(top);
+	while (fifo != NIL)
 	{
+		/* Get next page to visit */
+		top = linitial(fifo);
+		fifo = list_delete_first(fifo);
+
 		buffer = ReadBuffer(r, top->blkno);
 		LockBuffer(buffer, GIST_SHARE);
 		gistcheckpage(r, buffer);
@@ -917,12 +928,10 @@ gistFindPath(Relation r, BlockNumber child)
 			 */
 			ptr = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
 			ptr->blkno = GistPageGetOpaque(page)->rightlink;
-			ptr->childoffnum = InvalidOffsetNumber;
+			ptr->downlinkoffnum = InvalidOffsetNumber;
 			ptr->parent = top->parent;
-			ptr->next = top->next;
-			top->next = ptr;
-			if (tail == top)
-				tail = ptr;
+
+			fifo = lcons(ptr, fifo);
 		}
 
 		maxoff = PageGetMaxOffsetNumber(page);
@@ -934,48 +943,24 @@ gistFindPath(Relation r, BlockNumber child)
 			blkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
 			if (blkno == child)
 			{
-				OffsetNumber poff = InvalidOffsetNumber;
-
-				/* make childs links */
-				ptr = top;
-				while (ptr->parent)
-				{
-					/* move childoffnum.. */
-					if (ptr == top)
-					{
-						/* first iteration */
-						poff = ptr->parent->childoffnum;
-						ptr->parent->childoffnum = ptr->childoffnum;
-					}
-					else
-					{
-						OffsetNumber tmp = ptr->parent->childoffnum;
-
-						ptr->parent->childoffnum = poff;
-						poff = tmp;
-					}
-					ptr = ptr->parent;
-				}
-				top->childoffnum = i;
+				/* Found it! */
 				UnlockReleaseBuffer(buffer);
+				*downlinkoffnum = i;
 				return top;
 			}
 			else
 			{
-				/* Install next inner page to the end of stack */
+				/* Append this child to the list of pages to visit later */
 				ptr = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
 				ptr->blkno = blkno;
-				ptr->childoffnum = i;	/* set offsetnumber of child to child
-										 * !!! */
+				ptr->downlinkoffnum = i;
 				ptr->parent = top;
-				ptr->next = NULL;
-				tail->next = ptr;
-				tail = ptr;
+
+				fifo = lappend(fifo, ptr);
 			}
 		}
 
 		UnlockReleaseBuffer(buffer);
-		top = top->next;
 	}
 
 	elog(ERROR, "failed to re-find parent of a page in index \"%s\", block %u",
@@ -997,7 +982,7 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child)
 	parent->page = (Page) BufferGetPage(parent->buffer);
 
 	/* here we don't need to distinguish between split and page update */
-	if (parent->childoffnum == InvalidOffsetNumber || !XLByteEQ(parent->lsn, PageGetLSN(parent->page)))
+	if (child->downlinkoffnum == InvalidOffsetNumber || !XLByteEQ(parent->lsn, PageGetLSN(parent->page)))
 	{
 		/* parent is changed, look child in right links until found */
 		OffsetNumber i,
@@ -1016,20 +1001,21 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child)
 				if (ItemPointerGetBlockNumber(&(idxtuple->t_tid)) == child->blkno)
 				{
 					/* yes!!, found */
-					parent->childoffnum = i;
+					child->downlinkoffnum = i;
 					return;
 				}
 			}
 
 			parent->blkno = GistPageGetOpaque(parent->page)->rightlink;
 			UnlockReleaseBuffer(parent->buffer);
 			if (parent->blkno == InvalidBlockNumber)
-
+			{
 				/*
-				 * end of chain and still didn't found parent, It's very-very
-				 * rare situation when root splited
+				 * End of chain and still didn't find parent. It's a very-very
+				 * rare situation when root splited.
 				 */
 				break;
+			}
 			parent->buffer = ReadBuffer(r, parent->blkno);
 			LockBuffer(parent->buffer, GIST_EXCLUSIVE);
 			gistcheckpage(r, parent->buffer);
@@ -1050,7 +1036,7 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child)
 		}
 
 		/* ok, find new path */
-		ptr = parent = gistFindPath(r, child->blkno);
+		ptr = parent = gistFindPath(r, child->blkno, &child->downlinkoffnum);
 
 		/* read all buffers as expected by caller */
 		/* note we don't lock them or gistcheckpage them here! */
@@ -1119,7 +1105,7 @@ gistformdownlink(Relation rel, Buffer buf, GISTSTATE *giststate,
 
 		LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
 		gistFindCorrectParent(rel, stack);
-		iid = PageGetItemId(stack->parent->page, stack->parent->childoffnum);
+		iid = PageGetItemId(stack->parent->page, stack->downlinkoffnum);
 		downlink = (IndexTuple) PageGetItem(stack->parent->page, iid);
 		downlink = CopyIndexTuple(downlink);
 		LockBuffer(stack->parent->buffer, GIST_UNLOCK);
@@ -1147,7 +1133,7 @@ gistfixsplit(GISTInsertState *state, GISTSTATE *giststate)
 		 RelationGetRelationName(state->r), stack->blkno);
 
 	Assert(GistFollowRight(stack->page));
-	Assert(OffsetNumberIsValid(stack->parent->childoffnum));
+	Assert(OffsetNumberIsValid(stack->downlinkoffnum));
 
 	buf = stack->buffer;
 
@@ -1284,7 +1270,7 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 	tuples[1] = right->downlink;
 	gistinserttuples(state, stack->parent, giststate,
 					 tuples, 2,
-					 stack->parent->childoffnum,
+					 stack->downlinkoffnum,
 					 left->buf);
 	LockBuffer(stack->parent->buffer, GIST_UNLOCK);
 	UnlockReleaseBuffer(right->buf);

src/include/access/gist_private.h

@@ -218,14 +218,11 @@ typedef struct GISTInsertStack
 	 */
 	GistNSN		lsn;
 
-	/* child's offset */
-	OffsetNumber childoffnum;
+	/* offset of the downlink in the parent page, that points to this page */
+	OffsetNumber downlinkoffnum;
 
 	/* pointer to parent */
 	struct GISTInsertStack *parent;
-
-	/* for gistFindPath */
-	struct GISTInsertStack *next;
 } GISTInsertStack;
 
 typedef struct GistSplitVector
@@ -293,8 +290,6 @@ extern void freeGISTstate(GISTSTATE *giststate);
 extern SplitedPageLayout *gistSplit(Relation r, Page page, IndexTuple *itup,
 		  int len, GISTSTATE *giststate);
 
-extern GISTInsertStack *gistFindPath(Relation r, BlockNumber child);
-
 /* gistxlog.c */
 extern void gist_redo(XLogRecPtr lsn, XLogRecord *record);
 extern void gist_desc(StringInfo buf, uint8 xl_info, char *rec);